1. Field of the Invention
This invention relates to a method and apparatus for separation of two immiscible liquids, one aqueous and the other non-aqueous, such as water and oil, where one liquid is highly emulsified in the other.
2. Description of Prior Art
Oily water is generated at many points during oil production, for example, at the wellhead, several barrels of oily water are generated for every barrel of oil produced. This oily water, also called produced water, must be treated to obtain acceptable levels of oil-in-water before re-injection into the well or disposal, i.e. into the ocean. Ballast and bilge waters also represents a big challenge because of the strict regulations with respect to disposing of such effluents. In the Great Lakes in North America, the bilge water must attain levels below 5 mg/l of total oil & grease before disposal. Oil spills and contaminated groundwater can generate high volumes of oily water which must be treated before disposal. Furthermore, most industries generate varying amounts of oily water, the quantity of which depends on their production rate and process. This industrially produced oily water must be treated to reduce the fouling of instruments and equipment, to minimize interference with other processes and to reduce oil accumulation in unwanted areas thereby minimizing hazards.
The treatment of oily water can be achieved by several well-known and accepted techniques. However, each technique has limitations which depend on the characteristics of the oily water to be treated. Most of the processes of the prior art use Stoke's Law as a basis for design.
Stoke's Law explains that the rising velocity of an emulsion particle (such as oil) in a continuous phase (water) is proportional to the square of the diameter of the emulsion particle. Therefore, by doubling the diameter of a given emulsion particle, the particle's rising velocity is multiplied by four, and thus, the time required for the droplet to rise to a collection surface is also reduced. As the emulsion particles get smaller, a random molecular motion called Brownian movement, tends to keep the emulsion particles in suspension. Through random molecular collisions of the surrounding aqueous phase with the finely dispersed emulsion particles, the effect of gravity on the emulsion particles tends to be suppressed, so that settling is slowed or completely halted.
One of the most widely used systems to treat oily water is an oil/water separator, such as the API (American Petroleum Institute) separator, which can remove free-floating oil but cannot separate fine emulsions (particles less than 150 μm in diameter). Other well-known and widely used systems are corrugated plates interceptors and parallel plates interceptors, which are limited to oil emulsions where particle sizes are 30 μm or larger. The removal of oil emulsions where the diameter of the particles is less than 20 μm is very difficult, because in many cases they make up a high proportion of the total oil content, and it is impossible to reduce the level in the discharge to the permissible levels with conventional equipment. To enhance the rising velocity of such emulsions, other techniques are used, like coagulation/flocculation followed by a gravity separation system. In this process, chemicals are used to destabilize and coalesce the emulsions in order to make them larger, and easier to separate from the water. Coagulation/flocculation processes produce an oil contaminated with expensive chemicals.
Filtration-like separation processes are another category of oil removal processes, that are effective in removing very fine emulsions (diameter of 2 μm or more) as described by Benachenhou in WO/02/20115. Several media materials are used alone or together The most commonly used media are polymeric materials, however sand, anthracite, and clay have also been used as separation media. When sand, anthracite and clay are used they are produced with a particular form or shape. These filtration technologies are also limited, because of their sensitivity to the presence of viscous oils and/or suspended solids. It is not unusual that the materials used as a separation media become clogged with highly viscous oils or with suspended solids within 24 hours of operation, thus requiring replacement of the filtration media or backwashing with fresh or treated water, which results in even more oily wastes or more contaminated backwash liquids.
Additional patents of some background interest are the following: U.S. Pat. No. 3,738,492; U.S. Pat. No. 4,022,694 ; U.S. Pat. No. 4,213,863; U.S. Pat. No. 6,015,502; GB. 1,418,806; GB 1,517,715